This invention relates to an apparatus and method for measuring high transient isotropic pressure in the vicinity of an underwater explosion.
There are many devices currently used for obtaining pressure measurements. Pressure gauges generally fall into three broad categories based on principles of operation: (1) liquid column, (2) expansible-element gauges and (3) electrical pressure transducers. Generally, only the latter is adapted for measuring high transient pressures such as exist near an explosion. While there are several types within the pressure transducer family used for this purpose, one of the more common types is the piezoelectric pressure transducer. This type employs crystals which, when arranged in particular orientation and subjected to pressure, produce an electrical potential.
Blast pressures emanating from explosions are presently measured by piezoelectric gauges connected by coaxial cables to a recorder. This method, while in accepted use today, has several disadvantages. The piezoelectric crystals break at above about 200 megapascals (2 kbar or approximately 32,000 psi), far below the pressure at the surface of an explosive charge (about 150 kbar) The coaxial cables connected with the piezoelectric gauges generate noise when the explosion shock wave passes over them, and this detracts from true readings. Furthermore, most of the gauges in present use today are too large for measuring small explosions in tanks under laboratory conditions. The use of slowly responding types of pressure measuring devices such as liquid columns are, of course, totally out of consideration for measuring explosive pressures.
There is a present need for simple and inexpensive arrangements for measuring the high transient pressures in the vicinity of an explosion.
It has been known for sometime that the wavelength of the fluorescent radiation given off by some crystals (e.g., ruby) is dependent on pressure on the crystal. For example, the R.sub.1 red line of ruby shifts 0.365 .ANG./kbar in reponse to pressure on the ruby. (1 bar equals approximately 1 atmosphere pressure). This method, for example, has become the standard method for measuring pressure inside a diamond anvil high-pressure cell. A ruby chip within the cell is irradiated by focused blue laser light and the fluorescent light (red) given off by the ruby is fed into a spectroscope for wavelength shift measurement. This method is illustrated in FIG. 3 of the drawings of this application for background purposes and will be more fully described herein after.
This principle is discussed in articles entitled "Pressure Measurement Made by the Utilization of Ruby Sharp-Line Luminescence", Science, Vol. 176, Jan. 1972; "An Optical Fluorescent System for Quantitative Pressure Measurement in the Diamond-Anvil Cell", Journal of Applied Physics, Vol. 44, 1973; "Calibration of the Pressure Dependence of the R.sub.1 Ruby Fluorescent Line to 195 kbar", Journal of Applied Physics, Vol. 46, No. 6, June 1975; and "The Diamond Cell Stimulates High-Pressure Research", Physics Today, Sept. 1976.
The article (third article above) regarding spectroscopy at very high pressures gives the wavelength shift of the ruby R.sub.1 and R.sub.2 lines under hydrostatic pressures up to 200 kbar, as illustrated in FIG. 1 of the drawings herein. Isotropic pressure loading shifts the lines without changing their shapes, as shown in FIG. 2 of the drawings herein. Under non-isotropic conditions the line shapes are distorted as illustrated in FIG. 2a of the drawings herein.
There is discussed in the prior art methods of measuring temperature change. There is disclosed in U.S. Pat. Nos. 4,075,493 and 4,215,275 a method employing a length of optical fiber with a layer of phosphor material at one end of the optical fiber to be placed as a temperature probe at a heat source. The phosphor is illuminated by ultraviolet light to excite the phosphor to emit light which is conducted through the optical fiber to detection near or at the free end thereof. The relative intensity of two distinct wavelength radiation bands is an indication of the temperature of the phospher layer, and hence that of the surrounding environment.
An optical temperature transducer is disclosed in U.S. Pat. No. 4,288,159, and an optical temperature sensor is disclosed in U.S. Pat. No. 4,278,349 illustrating the ability of certain materials to change color in response to temperature change. Light of two different wavelengths is transported to the material via two optical fibers and transported back through another optical fiber to detectors for determining the change in absorption of the material.
The use of optical fiber waveguides for transporting light, including laser light, over long distances is rapidly developing in the art due in large part to low loss fibers now available on the market. The use of optical fibers in interferometric arrangements for detecting acoustic pressure changes in a liquid medium (e.g. water) is known. For example, see U.S. Pat. No. 4,162,397 issued July 24, 1979 to Joseph A. Bucaro et al, wherein light travels along two equal length paths, one of which is exposed to an acoustic pressure wave while the other is isolated therefrom. The optical fiber exposed to the pressure wave has introduced therein a phase shift of light passing therethrough which can be detected relative to that of light having pass through the isolated path which is not subjected by the pressure wave.
There are numerous methods known for detecting changes in temperature and transient pressure. The present system employs an arrangement not taught or suggested in the prior art, and it is inexpensive, rugged and ideally adapted to measure, without destruction, extremely high transient pressures such as those near an underwater explosion, not heretofore possible with piezoelectric-type transient pressure measuring devices.